1. Field of the Invention
One disclosed aspect of the embodiments relates to a speed detection apparatus that detects a detection mark, and a driving mechanism control apparatus that uses the speed detection apparatus.
2. Description of the Related Art
There have conventionally been offered a technology for highly accurately detecting a speed of a mobile object such as an intermediate transfer belt, and a technology for controlling the speed of the mobile object using the same. For example, there is known a method for attaching a scale member including a plurality of detection marks at constant pitches to the mobile object, detecting a signal from a sensor disposed oppositely to the mobile object, measuring periodical time of the marks based on the detected signal, and detecting the speed of the mobile object from the periodical time and the mark pitches. However, in this method, mark pitch accuracy on the mobile object exerts an influence on the measuring result, and a detection error becomes larger when the mark pitch accuracy becomes lower.
On the other hand, as a speed detection method (section speed detection method) which is not affected by the mark pitch accuracy, there is known a method for providing two sensors arranged in a relative moving direction. According to this speed detection method, a relative speed V is calculated by the following expression:V=L/TL TL: section time (time difference) required by the same detection mark to pass through the two sensorsL: predetermined distance between the sensors
As illustrated in FIG. 13, signals are detected from first and second sensors 70a and 70b, and section time (time or hour difference, hereinafter time difference) TL between the two detected signals is measured to detect a speed. The use of this section speed detection method eliminates a detection error caused by pitch accuracy reduction of detection marks 200 on a mobile object 100 such as an intermediate transfer belt, thereby enabling accurate speed detection. FIG. 13 illustrates a driven roller 31, a secondary transfer upper roller 41, a driving roller 80a, a mark detection circuit 80b, a section time calculation circuit 400, and a control unit 500.
As such a section speed detection system, the following belt speed control apparatuses in copying machines have been offered.
The belt speed control apparatus discussed in Japanese Patent No. 3344614 includes a plurality of detection marks arranged at predetermined pitches on a belt surface and two optical sensors arranged with a distance equal to that between the marks. With this configuration, a time difference between time of detecting the detection mark by the front (upstream side in a mobile object moving direction) optical sensor and subsequent time of detecting the detection mark by the rear (downstream side in the mobile object moving direction) optical sensor is calculated. Further, a belt speed is detected based on the time difference. Thus, the belt speed is detected by measuring time required by the same detection mark to pass between the sensors during belt movement.
In the belt speed control apparatus discussed in Japanese Patent No. 4429895, time periods of N marks are integrated while a distance between two optical sensors is set as an integral multiple of a mark pitch interval, and then a belt speed is detected by calculating time required by the same mark to pass between the sensors during belt movement.
However, in the belt speed control apparatuses discussed in Japanese Patent Nos. 3344614 and 4429895, if there is a mark failure such as a joint, damage, or stain in the detection mark, a reflected light amount decreases to generate a mark detection error such as mark detection omission or detection period fluctuation. Consequently, it is difficult to detect an accurate belt speed.
Referring to FIG. 14, since mark detection omission occurs in one of the mark detection circuits 80a and 80b, the mark detection circuit 80b will be described. That is, as illustrated in FIG. 14, when a mark detection error occurs, a belt speed is detected based on a time difference TNG different from the time difference of the same mark in passing between the two sensors.
In other words, while the section time (time difference) TL as illustrated in FIG. 13 should be detected in the nature of things, several fold time difference TNG is detected. As a result, the detected belt speed is slower than an actual belt speed.
Thus, when belt control is performed by using the belt speed including such an error, an excessive driving force may be instructed during belt driving, creating a possibility of great fluctuation in belt speed. In such a case, a position of toner transferred to the mobile object 100 such as an intermediate transfer belt may be greatly shifted to cause distortion or color misregistration of an output image. Thus, there is a possibility that erroneous detection of a detected speed will greatly affect driving mechanism control.
In the technology discussed in Japanese Patent No. 4429895, as described above, the distance between the two sensors is an integral multiple of the mark pitch. According to this technology, in detection of the section speed at which the same mark passes between the sensors spaced from each other by a predetermined distance, the passing time difference of the same mark is measured. In the case of a distance relationship between two sensors in which a plurality of mark cycles is present, and the mark cycle, it is difficult to identify the same mark at the two sensors.
To identity the same mark, a method for identifying the same mark may be employed, which counts the number of mark cycles which are present between the two sensors, with one sensor during the movement, and detects the mark with the rear sensor (downstream sensor) based on the counted value.
However, particularly as discussed in Japanese Patent No. 4429895, when the detection mark is disposed in the flexible mobile object and the distance between the two sensors is the integral multiple of the mark pitch, the number of mark cycles between the two sensors changes due to a load change on the flexible object or a change in ambient temperature. As a result, an identification error of the same mark easily occurs, creating a possibility of erroneous detection of a section speed.
Even if the flexible object is a rigid object, when a film scale having a detection mark formed on a base material of a film is attached to the rigid object, there is a possibility of mark cycle unevenness which occurs due to elongation or contraction caused by load fluctuation during the attachment. In this case, similarly to the aforementioned case, an identification error of the same mark easily occurs, creating a possibility of erroneous detection of a section speed.
Further, when a mark detection error occurs due to a joint, damage, or stain in one of the two sensors, the same mark cannot be identified, thus creating a possibility of a speed detection error. As a result, in a driving mechanism control apparatus using this speed detection apparatus, control errors will increase.